1. Field of the Invention
The present invention relates to a process for selectively producing chlorosilanes with an enhanced hydrogen content and, in particular, to an improved process for selectively providing enhanced yields of trichlorosilane utilizing oxygen promotion of silicon in a direct synthesis reaction between hydrogen chloride and silicon.
2. Description of the Prior Art
Chlorosilanes are conveniently prepared by the direct reaction of hydrogen chloride with silicon metal at elevated temperatures. The products of the reaction include: monochlorosilane, dichlorosilane, trichlorosilane, silicon tetrachloride and hydrogen as illustrated in the following generic equation for the reaction: EQU Si+HCl.fwdarw.H.sub.x SiCl.sub.4-x +H.sub.2
wherein x is an integer from 0 to 3. In general, chlorosilanes containing hydrogen tend to have a higher commercial value than silicon tetrachloride. Such compounds are especially useful in the synthesis of organosilicon compounds and organo functional chlorosilanes. Trichlorosilane is also a starting material for producing semiconductor grade silicon. The demand for organosilicon compounds and semiconductor grade silicon is increasing rapidly.
The direct synthesis reaction between silicon and hydrogen chloride to produce trichlorosilane was first reported in 1857. As practiced commercially today, the distribution of chlorosilanes in the product stream may typically be as follows:
______________________________________ Chlorosilane Amount (Weight %) ______________________________________ monochlorosilane (H.sub.3 SiCl) &lt;0.1 dichlorosilane (H.sub.2 SiCl.sub.2) &lt;0.5 trichlorosilane (HSiCl.sub.3) about 80-90 silicon tetrachloride (SiCl.sub.4) about 10-20 ______________________________________
Although the yield of trichlorosilane may be higher at the start of the reaction, it tends to decrease over time (see Example 1 below).
The direct synthesis of trichlorosilane by the reaction of silicon with hydrogen chloride is also described in the following prior art:
______________________________________ Country U.S. Pat. No. Issue Date ______________________________________ U.S. 3,148,035 September 8, 1964 U.S. 3,704,104 November 28, 1972 U.S. 4,130,632 December 19, 1978 Great Britain 945,618 January 2, 1964 Germany 1,102,119 March 16, 1961 ______________________________________
The direct synthesis reaction between hydrogen chloride and silicon, which is normally conducted at elevated temperatures, typically from 200.degree. to 500.degree. C., is highly exothermic. Highly exothermic reactions tend to cause hot spots within a reaction mix. Accordingly, it is essential to efficiently remove such heat to control the temperature of the reaction. Further, as the reaction temperature increases, it is well understood that the quantity of trichlorosilane produced decreases, while the content of silicon tetrachloride produced increases.
To control the excess heat of reaction it has been proposed to provide a stirred bed, an expanded bed an agitated bed or a fluidized bed reactor. Processes for producing trichlorosilane by reaction of silicon with hydrogen chloride employing a fluidized bed are disclosed in the above-listed U.S. and foreign patents. For example, in British Patent 945,618 trichlorosilane was produced in yields on the order of 90% by the reaction of silicon and hydrogen chloride in a fluidized bed.
In U.S. Pat. No. 4,092,446, issued May 30, 1978, a fluidized bed reactor is disclosed for producing trichlorosilane employing an internal heat exchanger to help dissipate the heat of reaction. In British Chem. Eng., Vol. 11, 9 (September 1966), pages 927-929, a fluidized bed process is disclosed for producing trichlorosilane at a reaction temperature of 300.degree. C. and at atmospheric pressure.
In general, the art has attempted to enhance the production of trichlorosilane during the direct synthesis reaction between silicon and hydrogen chloride by attempting to achieve proper temperature control in the reactor. Until now, such procedures have not been completely successful.
Oxygen, at concentrations of 1-500 ppm, was found to slightly decrease the relative selectivity of dimethyldichlorosilane in the direct reaction of methyl chloride with silicon metal; the decrease in selectivity increased as the oxygen concentration increased (J. Organometallic Chemistry, 84 (1975) 305-316).